1. Field of the Invention
This invention relates to a device and method for organizing optical fibers and the like at splice or slack points, and to the joining of fiber optic cables.
2. Background of the Invention
The handling of optical fibers requires special care. This is particularly true in splice enclosures. Due to their size, which can be as small as 0.2 mm in diameter, the handling of fibers is a problem. Transmission capabilities will be impaired if a fiber is bent beyond the critical radius, the point at which light is no longer totally contained in the core of the fiber. Furthermore, fibers are brittle and will break if bent beyond a minimum bending radius. The minimum bending radius here refers to that radius below which the probability of breakage of the fiber within a given time becomes high. Unconventional methods for the handling and storage of fibers must therefore be devised. While glass and silica (the materials used to make optical fibers) are in some respects stronger than steel, fibers normally do not possess this potential strength because of microscopic surface fractures which are vulnerable to stress and spread, causing the fiber to break easily. Thus the take-up of fiber slack (a bight in a fiber) in a closure presents a problem for multi-fiber cables, where individual fiber joints are required to facilitate rearrangements and repairs. Another problem is that of identifying individual fibers. In large multi-fiber cables each fiber must be readily identifiable for subsequent testing and repairs. Unlike copper where the insulation may be color coded, coding is difficult with individual optical fibers.
In an attempt to mitigate these problems, a standard splice enclosure with a central transverse bulkhead has been used. The individual fibers are spliced and are attached to the bulkhead for support. A disadvantage of this approach is that storage of slack in the fibers is not provided for. Furthermore, each of the fibers must be individually tagged for indentification purposes.
Another approach uses a ribbon type optical fiber arrangement where twelve fibers are fixed together side by side. Twelve of these ribbons are then stacked one on top of another to obtain a cable containing 144 individual fibers. The fibers are bulk spliced using an epoxy technique and the cable is placed in a standard splice enclosure. Disadvantages of the above approach are the lack of access to individual fibers and, again, no slack storage. A single fiber failure is impossible to repair, and the fiber must be taken out of service.
In other splicing arrangements all the fibers in a cable are looped within the same retainer or fiber slack is stored on spools. In either case identification, repair or splice work of individual fibers is extremely difficult without a major shuffle in the splice enclosure. This is undesirable as transmission capability can be affected in working fibers as they are moved.
Hutchins U.S. Pat. No. 4,226,853, describes an organizing device for optical fibers which comprises a plurality of stacked tray-like supports having partially turned-up edges, with each separately hinged at one side thereof to a carrier. Each tray-like support is adapted to retain a looped fiber portion, and has a width at least equal to twice the minimum bending radius specified for that fiber.
Fiber optic communications cables generally include some form of strength and/or sheath member to carry the strain involved in installing the cables in ducts or laying them underwater, since the fibers are not capable of bearing such strains themselves. When two fiber optic cables are spliced there should, therefore, be means provided at the splice to transfer the strain from one cable strength member to the other cable strength member.
In U.S. Patent Application Ser. Nos. 339,275 filed Jan. 15, 1982 and 381,825 filed May 25, 1982, which are assigned to the assignee of the present application and which are incorporated herein by reference, there is disclosed a device and method for organizing a bight in an optical fiber, such as at a splice between two fiber optic cables. The method comprises wrapping the bight about a cylindrical core to form a helix or a pair of helices of opposite handedness.
The technique described in those applications has the advantage that the radius of the core can be less than the minimum bending radius of the fiber while still allowing storage of fiber in such a way that it is bent through a radius greater than the minimum bending radius. A disadvantage, however, is that when the core radius is small, an undesirably long core may be required to store an adequate length of fiber. A further disadvantage is that the technique is not readily applicable to butt splicing of fiber optic cables, but only to in-line splicing.